It has long been a goal of membrane research to prepare membranes capable of economically separating oxygen from nitrogen, where both are contained in air. Potential applications of such membranes are many, including for example oxygen-enriched air for internal combustion engines, oxygen-enriched air for medical use, and generation of oxygen-free nitrogen for inerting the atmosphere above flammable fuels. Especially sought attributes of membranes are high oxygen selectivity combined with high oxygen permeation rate. Polydimethylsiloxanes, also known as silicones, are known to be selectively permeable to oxygen versus nitrogen, with a selectivity ratio of about 2. The polydimethylsiloxanes also show higher permeabilities to gases in general than almost all other materials. This characteristic has made them of special interest in oxygen enrichment applications. However, the poor physical strength characteristics of siloxane polymers in general and polymethylsiloxanes in particular result in the need to employ rather thick films of the polymers. Gas throughput rates are consequently low.
One method of overcoming this drawback has been to form thin, crosslinked films upon microporous substrates. An attractive approach to achieve this is by use of gas plasmas. Plasma polymerization has the ability to deposit coatings that are uniform, pinhole-free, highly adherent, and ultrathin, i.e., less than 2 micrometers in thickness. Plasma polymerization via gas plasmas has been used to prepare thin coatings on the surfaces of substrates, particularly microporous substrates, as a means of preparing semipermeable membranes with useful properties. This has been commonly achieved using "low temperature" or "cold" plasmas, which are generated at reduced gas pressures under glow discharge conditions. Radiofrequency (RF) generating electrodes are typically used to generate low temperature glow discharges.
The preparation of coatings from plasma polymerization of siloxane monomers has been known for some time. In the book "Plasma Polymerization" by H. Yasuda (Academic Press, 1985), data on plasma polymers from hexamethyl disiloxane and tetramethyl disiloxane dating back to 1971 are cited. U.S. Pat. Nos. 4,410,338 and 4,594,079 disclose the preparation of gas separating membranes wherein a layer of hexamethyl disiloxane plasma polymerizate is formed by glow discharge methods. U.S. Pat. No. 4,696,686 discloses oxygen separating membranes prepared from plasma deposition of a hexamethyl disiloxane polymerizate modified with fluorine-containing moieties on a porous polypropylene substrate. U.S. Pat. No. 5,002,652 discloses the deposition of an oxygen-permeable polysiloxane coating from a hexamethyl disiloxane gas plasma onto an electrode to form a sensor. In these disclosures, plasma reaction times were routinely reported to be in the range of 30 to 40 minutes. Mention is omitted in these disclosures regarding the difficulty of obtaining high molecular weight polymeric compositions from hexamethyl disiloxane in short reaction times, and in satisfactorily bridging pores in the surface of porous substrates in short reaction times.
It is in fact difficult to obtain suitable coatings for gas separations in a short plasma treatment of 1 to 5 minutes, or less. According to Table 6.13 of page 109 of the book cited above, a polymer deposition rate from a continuous discharge plasma of hexamethyl disiloxane has been observed to be 0.223.times.10.sup.-8 g/cm.sup.2- hour. This rate is approximately equivalent to a thickness deposition rate of about 4 angstroms per second. This rate is relatively slow, such that build-up of a contiguous film that bridges over pores in a porous substrate is problematic. Consequently, long plasma exposure times on the order of 30 minutes are required to deposit a film of sufficient thickness and contiguity to fully cover pores in a porous substrate such as microporous polypropylene sheet or hollow fiber. Only when all surface pores of a substrate have been bridged with a coating of sufficient thickness to withstand transmembrane operating pressures, can membranes made thereby exhibit good permselectivity toward gases. Unfortunately, this approach, utilizing long plasma polymerizate exposure times, results in thick, dense polymerizate deposits characterized by lower than desired gas fluxes through intact membranes. A faster rate of polymerizate deposition would be a highly desirable trait, one that is not met via the disiloxane plasma polymerization methods disclosed above.
In addition to the difficulty of obtaining a sufficiently rapid deposition rate of plasma polymer, one is also faced with differences in the form of the porous substrates to be coated, and difficulties arising therefrom. In particular, it is relatively easy to plasma coat a flat sheet of a film fixed in position and facing a gas plasma containing a disiloxane. It is more difficult to coat a hollow fiber substrate located in a stationary position, such as described in U.S. Pat. No. 4,410,338, because deposition is not typically uniform in all three dimensions in a gas plasma. Thus, a hollow fiber substrate may have regions of thick deposition and thin deposition adjacent to one another, differing only in the arc of the surface and its distance from an electrode surface. Furthermore, a commercially attractive coating operation must be able to handle a moving length of substrate, as in a continuous coating operation. Gas plasmas have the effect of thermally heating the substrate being coated. If the thermal heating is too intense, the substrate will undergo deformation in handling. For flat sheet substrates, sheet thickness and reinforcing fabric backings can be generally modified so as to resist substrate deformation, as the sheet is pulled through a plasma reaction zone. On the other hand, for porous hollow fiber substrates, such as made of porous polypropylene, thermal heating and potential substrate deformation during pulling become critical factors that severely limit the length of exposure that can be tolerated.
It is indeed next to impossible to adequately move a porous polypropylene fiber of the type used in Example 20 of U.S. Pat. No. 4,410,338 through a plasma treatment of the type disclosed because of thermal heating and fiber deformation. The results described in Example 20 of the patent cited immediately above were obtained on nonmoving fibers supported on a fixed support frame. The difficulty in obtaining uniformity and depth of coating in this hollow fiber example were evident in that the oxygen versus nitrogen selectivity of the polypropylene hollow fiber was 2.3, i.e., no better than the lowest observed selectivity value observed for flat sheet examples coated with a like composition under the same plasma polymerization conditions. Thus, the need for a faster rate of polymerizate deposition, unmet by the above-mentioned disiloxane plasma polymerization methods, is crucial to the ability to rapidly coat a hollow fiber in a time period wherein the fiber is not softened or caused to distort by input of too much thermal energy by the plasma itself.
In addition to the need for a higher polymerizate deposition rate, plasma polymerizate deposited from a hexamethyl disiloxane gas plasma is marked by tacky or blocking (i.e., self-sticking) surface characteristics. This behavior indicates the presence of excessive amounts of low molecular weight components. Fully alkylated disiloxanes such as hexamethyl disiloxane do not very readily polymerize to high molecular weight polymers, even by plasma polymerization processes. This is particularly noticeable at short plasma polymerization exposure times of 5 minutes or less, where the polymerizate deposits are tacky or blocky. If the polymerization is continued for much longer times (viz. 30 to 40 minutes) to generate polymerizate deposits that are handleable, such long plasma polymerization exposure times have the effect of lowering gas permeabilities of resulting composite semipermeable membranes by reason of the deposition of thicker, denser polymerizate coatings. Tackiness of a coating deposited on microporous hollow fiber substrates engender a host of handling difficulties, in that the coated fibers, for example, will stick to one another. Not the least of such handling difficulties is the disruption of ultrathin plasma coatings when fibers are pulled apart or moved relative to each other, causing formation of leaks. Another need exists therefore to improve the plasma polymerization of fully alkylated disiloxanes such that deposited polymerizate coatings are dry, i.e., not tacky or self-adhering.
U.S. Pat. No. 4,824,444 discloses the preparation of gas separation membranes with improved selectivity from tetramethyl disiloxane and its tetraalkyl analogs. This monomer group is characterized by having a hydrogen atom bonded to each silicon atom in the monomer. The hydrogen-silicon bond is considerably weaker than a alkyl-silicon bond, such that tetramethyl disiloxane and its tetraalkyl analogs polymerize readily by gas plasma techniques. Residence times of a porous substrate in a zone of tetramethyl disiloxane gas plasma were reported as being on the order of 19 seconds; yet suitable gas separating barrier layers were deposited. Unfortunately, this type of monomer is inordinately expensive relative to hexamethyl disiloxane. The high monomer expense severely negates its economic viability in the preparation of gas separation membranes.
In summary, therefore, a need remains in the area of plasma-deposited polysiloxane polymerizates for a method of rapidly generating such deposits using an inexpensive disiloxane monomer such as hexamethyl disiloxane in a manner not requiring excessively long plasma exposure times. In addition to this need for a greatly enhanced polymerizate deposition rate, a need also exists for a method of achieving high molecular weight polymerizate polymers from fully alkylated disiloxanes that do not have tacky or self-adhering surface characteristics, so that resulting coated articles can be properly and reasonably handled.